A technique for further increasing the data storage density of an optical disc by using a solid immersion lens (SIL) and near-field light has been proposed. A typical SIL is a high-refractive-index lens, of which the shape is defined by cutting out a portion of a spherical lens. An SIL is inserted between a condenser lens and an optical disc to produce near-field light at the bottom.
To read and write data from/on an optical disc using an SIL, the SIL and the surface of the optical disc need to be so close to each other that the near-field light can reach the surface of the optical disc. In addition, by performing a so-called “gap control” to keep the gap between the SIL and the optical disc surface constant, the light beam spot on either the disc surface or the information storage layer should maintain a predetermined size.
Such a gap control is carried out by taking advantage of the property that the intensity of the light returning from an SIL varies its level according to the magnitude of the gap between the SIL and the given optical disc. Patent Document No. 1 discloses an exemplary optical information apparatus that performs such a gap control. Specifically, such a gap control is carried out by comparing a voltage converted from the intensity level of the light returning from the SIL to a reference voltage representing a desired gap and by getting the SIL driven by an actuator such that the difference between these two values becomes as small as possible. The magnitude of the gap can be varied by changing the reference voltages.                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2002-319160        